This invention relates to microscopes which are used in infrared (IR) sample analysis, and particularly to the use of such microscopes in the reflectance mode, i.e., in situations in which the incoming modulated radiation is reflected at the sample. Such reflection may be caused by the sample itself, or by an adjacent reflecting surface which causes radiation to pass through the sample.
As disclosed in an earlier application of the same inventor, having a common assignee, identified as application Ser. No. 907,995, filed Sep. 16, 1986, a relatively high radiation throughput for reflectance in an FTIR microscope can be attained by using a fully reflecting "injection" mirror to direct haIf of a post-interferometer radiation beam through the objective lens toward the sample. The returning IR beam reflected at the sample passes again through the objective lens, bypasses the injection mirror, and reaches the detector. The collimated beam entering the microscope from the interferometer is focused and then recollimated by two confocal parabolic mirrors before the beam reaches the injection This system provides four image planes, (in which the radiation is brought to a focus) one at the sample, one at the detector, a third between the objective lens and the detector, and a fourth between the two confocal parabolic mirrors.
In any infrared microscope, it is important to be able to limit the area being analyzed, while providing a visible means for positively identifying the extent of this area. In transmission microscopy, this is generally accomplished quite simply by locating a variable size (adjustable) field stop, or iris, in a plane which contains a magnified image of the sample, and which is located between the objective lens and the movable mirror used to direct the optical radiation transmitted through the sample either to the eyepiece or to the infrared detector.
In reflectance microscopy, the use of the approach outlined above introduces both an additional benefit and a problem. The benefit arises from the fact that the incident and reflected light paths are coincident in the plane of the adjustable field stop. The adjustable field stop thus acts to limit both the area illuminated by the incident radiation and the area viewed by the detector. This "double aperturing" serves to reduce the diffraction spread caused by the finite aperture stop (throughput-limiting aperture). In other words, the combination of an image-limiting field stop which is passed through by entering radiation (between the source and the sample) with an image-limiting field stop which is passed through by exiting radiation (between the sample and the detector) reduces the stray light due to the diffraction effect, and minimizes its negative effect on the image at the detector.
The problem associated with using an adjustable field stop common to both the incident and reflected beams results from the fact that radiation reflected from the "back" surface of the adjustable field stop will reach the IR detector, causing a significant "stray light" offset in the measured spectrum.
Light scattering from the rear of the adjustable field stop could be avoided by injecting the incident beam below the adjustable field stop. However, as discussed in the previous application, this may not be convenient in a particular microscope design. In addition, unless the field of view of the incident beam is limited by an adjustable field stop somewhere else in the optical train, an excess of IR radiation will be present in the region of the sample, and perhaps in the objective lens (depending on where the injection takes place). Some of this undesired radiation may be scattered into the system field-of-view, again leading to detected stray light.
A definition of terms relating to radiation "stops" may be useful at this point. As explained in the textbook "Fundamentals of Optics", by Jenkins and White, a stop (i.e., an element which limits the radiation passing through a given position) may be either an "aperture stop" or a "field stop". An aperture stop determines the amount of light reaching any given point in the image, and therefore controls the brightness of the image. A field stop determines the extent of the object, or the field, that will be represented in the image.